1. Field of the Invention
This invention relates to a new method of "bleaching" deposits of metallic silver, and in a particular aspect, it relates to photographic processes and products employing the same.
2. Description of the Prior Art
It is well-known that precipitated metallic silver produced by reduction from solution may vary in color depending upon the particle size of the silver crystals. For example, the reduction of silver nitrate in aqueous alkaline solution yields so-called high covering power black silver composed of small crystals having a particle size up to about 0.1 micron. Reduction in the absence of alkali yields gray silver having a larger particle size up to about 100 microns. In photographic processes, the silver produced by development of an exposed photosensitive silver halide emulsion, though generally filamentary in nature, also may vary in color. Normally, the image silver obtained by chemical development under alkaline conditions has a relatively high covering power and comprises black silver or a near black form.
Customarily, bleaching of precipitated silver and of developed, i.e., image silver is achieved by treating the metallic silver deposit with an oxidizing agent in the presence of a soluble halide whereby the silver is converted to a pale or colorless silver salt, which, if so desired, may be removed by washing or by converting to a soluble salt, i.e., "fixing" and washed out. Commonly employed for this purpose are bleaching baths comprising potassium ferricyanide and a soluble halide, such as, an alkali metal halide, e.g., potassium bromide wherein the metallic silver is converted to silver bromide crystals according to the following equation: EQU [Fe (CN.sup.-.sup.-.sup.-).sub.6 ] + Ag.sub.o + Br.sup.- .fwdarw. [Fe(CN.sup.-.sup.-.sup.-) .sub.6 ] + [Ag.sup.+Br.sup.-]
Other soluble halides that have been employed include ammonium bromide, potassium iodide and phosphonium iodides.
The present invention is concerned with a method for reducing the covering power of metallic silver which finds particular utility in photography as a bleach for decreasing the optical transmission density of developed, e.g., negative image, silver.